When trying to read and analyze X-ray images it’s important to remember that the density of the scanned item determines the color of the X-ray images. When the material is denser, it appears darker.
Also, the color settings of different machines may differ, and although most items will appear the same way in different bags, they may appear differently in other bags.
Blue represents dense materials, such as metal, hard (dense) plastics, and different parts of weaponry. When the material is very dense, the blue may get darker to the point of black. Metal boxes or wires, for instance, will appear blue or black, or sometimes a mix of the two colors.